This is a hard case to argue... As some people said there is no "BEST" OS. However, I would say most of the windows NT series(NT, 2000, XP, and future ones)
I have tried a few other OS's; Linux, Solaris, Macs, BeOS... Many can be more stable and sometimes faster than windows systems, but they all have one major fault... Compatability. EVERYTHING is based around windows. Applications, games, drives...you name it.
Now, if all you need is a sable computer to do a single function, like type...I'd say go with some other OS. Beleive me, others out there have many less problems than windows! Hehe, why else do you think TiVO's are Linux based ;P I also hear some accesspoints are(or mine has been thought to be Linux based)